The present invention relates to the detection of optical sensors by means of measurement of information relative to a signal intensity and a signal modulation phase shift.
The market of inexpensive, smart and disposable intelligent labels, RFID and smart tags is rapidly growing. The most demanding properties to be sensed by such devices are temperature, pressure, moisture, pH, gas concentrations (CO2, CO, NH4, O2, and others) and the concentration of specific chemical ions (ammonia, etc). The basic challenge today consists in bringing on the market a smart solution both on the sensor as on detecting device side. In a most general approach, the sensing element needs to be included into the detection area in such a way that is able to sense the interesting propriety (gas, pH, . . . ) while still able to be remotely activated and interrogated by the distant measurement unit.
One of the future customers of such technology is the packaging industry, where several goods, depending to their nature, need to be packaged under well defined and controlled conditions. Thus, chemical, pharmaceutical and electronic industries are frequently confronted to the problem of exposure to factors like temperature, oxygen or moisture, which excess is leading to the premature degradation of packaged good. The food packaging industry is typically struggling with too high concentration of oxygen and to high temperature.
There are several other applications, where remote, optical evaluation of carbon monoxide, dioxide, ammonia or specific chemical ions is necessary for the general safety or product shelf life reasons. A good example of today's industrial needs is the oxygen sensing for food packaging industry that is currently employing MAP (Modified Atmosphere Packaging) packaging foils to guarantee a constant and low oxygen concentration to the packaged goods. Here, the solution will consist on placing inside such package, or placing it directly on the packaging foil, an oxygen-sensitive element able measure the 02 concentration and communicate this information optically or by RF to the detector unit.
Very strong price pressure, small size requirement and real need of disposable sensors eliminate a variety of “smart tags” communicating remotely with detection or analyzing unit by RF.
The solution of choice in this case consist on disposing inside the package of chemical compound whose light emission properties are directly depending to the oxygen concentration. This can be easily achieved if using one of several possible easily, optically excitable organometallic complexes (transition metal-organic complexes, preferably highly aromatic compounds like porphyrins, etc., for which the oxygen particles are fluorescence quenchers. Thus, the sensing compound or luminescence probe, after its prior excitation by a remote illumination at a specific wavelength, will emit an optical signal (fluorescence) which decay in time is directly informing of temperature and of oxygen concentration. This quencher (here: O2) to signal decay relation is described by the Stern-Volmer relation.
The measurement can relay on the emitted light intensity or on the emission (fluorescence) life time. In the first case, the dependency to the luminescence probe concentration and its purity constitutes a strong disadvantage. The measurement based on fluorescent signal decay, fluorescent modulation phase shift or polarization type and change of emitted light are all not depending to this. Unfortunately, a fluorescence intensity and life time of almost all interesting compounds is not only depending to the concentration of specific fluorescence quenchers but also to temperature.
There is however, a possibility to use another, optically activated compound, the fluorescence intensity or decay time of which is only depending on the temperature. Such compound, just employed as standalone, gives an excellent solution for the disposable and remote temperature sensor, while used along with a quencher-dependent compound, offers internal temperature calibration of the Stern-Volmer relationship linking the quencher concentration, temperature and fluorescence decay. Oregon Green-488 fluorescent dye can be employed for this purpose along with one of several known oxygen-sensitive compounds like Ru(II)[dpp(SO3Na)2]3)Cl2, Ru(II)(dpp), PdTCPPP, PtOEP or other Cr, Mn, or transition metals-organic complexes. An important characteristic of using fluorescence probes is that they are reversible within a very short time period to encompass rapid temperature or atmosphere changes.
Current technology of excitation/analyzing units employs lock-in analyzers, modulated signal generators and photomultipliers. These devices are very complex and have a very large size. Downsizing of such equipment to handheld devices required in several applications is not possible both from a technical and a commercial (high prize) point of view.